CMOS (complementary metal oxide semiconductor) solid-state imaging devices are known to be classified into a front-illumination type and a rear-illumination type. A solid-state imaging device of either of the two types includes a pixel region in which a plurality of unit pixels are formed on a semiconductor base substrate and each of the unit pixels is formed of a photodiode that works as a photoelectric converter and a plurality of pixel transistors.
In a front-illumination solid-state imaging device, the front surface of a substrate on which a multilayer wiring layer is formed works as a light receiving surface, and light is incident on the front surface of the substrate.
In a rear-illumination solid-state imaging device, the rear surface of the substrate that faces away from the front surface of the substrate on which a multilayer wiring layer and pixel transistors are formed works as a light receiving surface, and light is incident on the rear surface of the substrate.
The photodiodes are isolated from each other by a device isolating region formed of an impurity diffusion layer. Further, the multilayer wiring layer, in which a plurality of wiring lines are disposed, is formed via an interlayer insulating layer on the front surface of the semiconductor base substrate, on which the pixel transistors are formed.
In a front-illumination solid-state imaging device, the wiring lines are formed in regions other than the photodiodes. On-chip color filters and microlenses are sequentially formed via a planarization layer on the multilayer wiring layer. The on-chip filters are formed, for example, of an array of red (R), green (G), and blue (B) filters.
In a rear-illumination solid-state imaging device, the wiring lines can be formed irrespective of the positions of the photodiodes. An insulating layer, on-chip color filters, and microlenses are sequentially formed on the rear surface of the semiconductor base substrate, which works as the light receiving surface of the photodiodes.
In a rear-illumination solid-state imaging device, since the multilayer wiring layer does not constrain light from entering the photodiodes in any manner, each of the photodiodes can be provided with a large opening. Further, the distance from the photodiodes to the microlenses can be shortened, as compared with that in a front-illumination solid-state imaging device. Shortening the distance can improve the ability of the microlenses to collect light, whereby obliquely incident light can also be efficiently introduced. As a result, the sensitivity of the solid-state imaging device can be increased.
To improve the ability of the microlenses to collect light, for example, the curvature of each of the microlenses may be increased, or the refractive index of the material of which the microlenses are made may be increased (see JP-A-2007-53318, JP-A-1-10666, JP-A-2008-277800, and JP-A-2008-9079).